VISAR systems are commonly used to study large changes in velocity by measuring the Doppler shift of laser light reflected from a moving object. The term VISAR has come to describe a system that includes the laser, delivery optics, interferometer, collection optics, and recording system. In a VISAR system, laser light is delivered to a target. Reflected or scattered light is collected and delivered to an interferometer for analysis and then to a recording system. The distinguishing characteristic of a VISAR system is the use of a wide angle, unequal-path Michelson interferometer to analyze the spectrum of the Doppler-shifted light that is reflected from an extended area on the target and often relayed to the interferometer via multimode optical fiber.
In a conventional Michelson interferometer, collimated light incident on a beamsplitter is directed along two paths to mirrors that reflect the light back to the beamsplitter, where the beams are recombined and interfere. If the light is well collimated and approximates a plane wave, the wavefronts will overlap well and can produce a high degree of fringe contrast even with significant delay in the interferometer. If the incoming wavefront is not planar but instead diverges from a point source or consists of a complex ensemble of diverging wavefronts produced by collimating light from an extended source, the phase of the interference will vary across the output beam and the overall extinction will be poor unless the path lengths in the interferometer are exactly equal.
To use a Michelson interferometer as a spectrometer for Doppler shift measurements, it is necessary to introduce a delay into one of the paths such that the resulting fringe spacing is appropriate for the spectral shift being measured. To achieve good extinction, additional optics must be added to the basic Michelson interferometer so that the recombined wavefronts match well in spite of the unequal paths. This is the essence of the wide angle Michelson interferometer used in VISAR. The VISAR, by its very nature as a wide field Michelson interferometer, equalizes the beam divergence over two optical paths of different lengths, thereby allowing the delayed beam to interfere efficiently with the undelayed beam. This equalization can be accomplished either by inserting thick windows of high index material into the delay leg and placing the cavity mirror at the correct distance to satisfy the wavefront matching requirements, or by the use of relay lenses to image the mirrors within the cavity. The glass windows approach is the most common VISAR arrangement. However, traditional VISAR design does not address the problem of efficient fiber-to-fiber coupling or imaging through the interferometer, in which the image size as well as the numerical aperture must be preserved.
Light from an extended source, such as a fiber array, can be collimated by a lens, relayed through an interferometer, and re-imaged by another lens, as described in U.S. Pat. No. 5,870,192 and U.S. Pat. No. 5,481,359. Light from all the fibers in the input array is collimated into a single beam that is not resegregated until the beam is reimaged onto the output array. Such a VISAR can be considered as an image relay system with no field elements. For most practical systems, the relay lenses are separated by a distance considerably larger than their focal lengths, which results in an increase of the numerical aperture and/or size of the image. For a system using 1:1 imaging to couple identical fiber arrays through a typical interferometer, the increased numerical aperture represents a significant loss of efficiency. The numerical aperture can be reduced by magnification, but larger fibers, which may not be compatible with small detectors, must then be used in the output array.
The divergence problem described above can be avoided by putting the collimating lenses in an afocal arrangement so that they are separated by the sum of their focal lengths. However, this results in an excessively long and large diameter optical path and is not generally practical.
The solution described in the present invention is to treat the problem as a system of relay lenses and use a field element to correct the divergence of the beam. In this approach, the fiber arrays are imaged at the cavity mirrors within the interferometer and the cavity mirrors are modified to also function as field elements to relay the pupil image. The resulting image of the input array at the output plane is precisely matched in size and numerical aperture to the output array, resulting in efficient coupling. Some other benefits of this approach are that it allows a larger field of view through the same diameter optics and reduces crosstalk by reducing aberrations. The larger field of view also allows larger fiber separation which enables improvements in array construction while further reducing crosstalk.
There are several benefits to the VISAR system that result from these interferometer improvements. Improved time resolution is a consequence of the improved imaging performance. Most existing VISAR systems use photomultipliers that have large detection areas and are very sensitive but are limited to approximately 1 nanosecond response time. Faster detectors are available in the form of photodiodes and streak cameras, but both of these require small sensing elements to achieve high speed. The improvements in VISAR interferometer optics provide efficient coupling even with small optical fibers that are compatible with fast detectors.
The high optical efficiency of the improved VISAR interferometer reduces the size and cost of the laser sources that are required. Laser power is often a limiting factor in designing an experiment using VISAR, particularly when high speed detectors are used. By raising the optical efficiency of the interferometer, the laser power required is considerably reduced.
The field elements in the improved VISAR eliminate vignetting and, in combination with the CCTV lenses, allow a much larger source than is usually possible. This allows more channels to be relayed through a given size of optics. It also allows more space between the fibers, which in turn allows more precise fiber array design. This, in turn, provides improved coupling and reduced crosstalk.
Since the fibers are imaged at the cavity mirrors, alignment of the optical system is simplified. The images of the fibers from all of the arrays are readily viewed at the cavity mirrors. Rotation, focus, magnification, and position can easily be adjusted to overlap the fiber images.
It is possible to split the delay leg so that a subset of the channels has a delay that is different from the other channels. Since the beams generated by the individual fibers in the array are imaged at the cavity mirror, they are spatially separate over several inches of the optical path near the mirror. A mirror can be placed to intercept one or more rows of beams and direct them to an alternate cavity mirror that provides a different delay. In this way, the improved VISAR can perform the function of two separate VISARs of the conventional design.
In summary, the use of intracavity imaging and field elements makes possible a number for significant improvements in VISAR system performance.